Skip to main content
Redhat Developers  Logo
  • Products

    Featured

    • Red Hat Enterprise Linux
      Red Hat Enterprise Linux Icon
    • Red Hat OpenShift AI
      Red Hat OpenShift AI
    • Red Hat Enterprise Linux AI
      Linux icon inside of a brain
    • Image mode for Red Hat Enterprise Linux
      RHEL image mode
    • Red Hat OpenShift
      Openshift icon
    • Red Hat Ansible Automation Platform
      Ansible icon
    • Red Hat Developer Hub
      Developer Hub
    • View All Red Hat Products
    • Linux

      • Red Hat Enterprise Linux
      • Image mode for Red Hat Enterprise Linux
      • Red Hat Universal Base Images (UBI)
    • Java runtimes & frameworks

      • JBoss Enterprise Application Platform
      • Red Hat build of OpenJDK
    • Kubernetes

      • Red Hat OpenShift
      • Microsoft Azure Red Hat OpenShift
      • Red Hat OpenShift Virtualization
      • Red Hat OpenShift Lightspeed
    • Integration & App Connectivity

      • Red Hat Build of Apache Camel
      • Red Hat Service Interconnect
      • Red Hat Connectivity Link
    • AI/ML

      • Red Hat OpenShift AI
      • Red Hat Enterprise Linux AI
    • Automation

      • Red Hat Ansible Automation Platform
      • Red Hat Ansible Lightspeed
    • Developer tools

      • Red Hat Trusted Software Supply Chain
      • Podman Desktop
      • Red Hat OpenShift Dev Spaces
    • Developer Sandbox

      Developer Sandbox
      Try Red Hat products and technologies without setup or configuration fees for 30 days with this shared Openshift and Kubernetes cluster.
    • Try at no cost
  • Technologies

    Featured

    • AI/ML
      AI/ML Icon
    • Linux
      Linux Icon
    • Kubernetes
      Cloud icon
    • Automation
      Automation Icon showing arrows moving in a circle around a gear
    • View All Technologies
    • Programming Languages & Frameworks

      • Java
      • Python
      • JavaScript
    • System Design & Architecture

      • Red Hat architecture and design patterns
      • Microservices
      • Event-Driven Architecture
      • Databases
    • Developer Productivity

      • Developer productivity
      • Developer Tools
      • GitOps
    • Secure Development & Architectures

      • Security
      • Secure coding
    • Platform Engineering

      • DevOps
      • DevSecOps
      • Ansible automation for applications and services
    • Automated Data Processing

      • AI/ML
      • Data Science
      • Apache Kafka on Kubernetes
      • View All Technologies
    • Start exploring in the Developer Sandbox for free

      sandbox graphic
      Try Red Hat's products and technologies without setup or configuration.
    • Try at no cost
  • Learn

    Featured

    • Kubernetes & Cloud Native
      Openshift icon
    • Linux
      Rhel icon
    • Automation
      Ansible cloud icon
    • Java
      Java icon
    • AI/ML
      AI/ML Icon
    • View All Learning Resources

    E-Books

    • GitOps Cookbook
    • Podman in Action
    • Kubernetes Operators
    • The Path to GitOps
    • View All E-books

    Cheat Sheets

    • Linux Commands
    • Bash Commands
    • Git
    • systemd Commands
    • View All Cheat Sheets

    Documentation

    • API Catalog
    • Product Documentation
    • Legacy Documentation
    • Red Hat Learning

      Learning image
      Boost your technical skills to expert-level with the help of interactive lessons offered by various Red Hat Learning programs.
    • Explore Red Hat Learning
  • Developer Sandbox

    Developer Sandbox

    • Access Red Hat’s products and technologies without setup or configuration, and start developing quicker than ever before with our new, no-cost sandbox environments.
    • Explore Developer Sandbox

    Featured Developer Sandbox activities

    • Get started with your Developer Sandbox
    • OpenShift virtualization and application modernization using the Developer Sandbox
    • Explore all Developer Sandbox activities

    Ready to start developing apps?

    • Try at no cost
  • Blog
  • Events
  • Videos

How to debug where a function returns using LLDB from the command line

September 11, 2019
Konrad Kleine
Related topics:
LinuxC, C#, C++

Share:

    I often find myself in a situation when I want to know where a function returns. There's no need to know the return value, as this may be the same for multiple code paths (e.g., nullptr if something went wrong). It is embarrassing, but I sometimes have put fprintf(stderr, "T1"); in my code just to follow which path the execution took. Needless to say, this behavior requires manual editing and recompilation and should be avoided if possible.

    Here's a way to elegantly debug where a function returns using lldb from the command line.

    Consider this test.cpp program and all you want to do is find out where the function foo returns:

    int foo(int argc) {
      switch (argc) {
      case 1:
        return 1;
      case 2:
        return 2;
      case 3:
        return 3;
      }
      return -1;
    }
    
    int main(int argc, char *argv[]) { return foo(argc); }
    

    Note that there are five return statements in this code, but we only want to know which of the four inside of foo are being hit.

    Let's start by compiling the above program with debug symbols:

    clang -g test.cpp

    To get to know where foo returns, you can run the following command.

    lldb -b -o "br set -X foo -p return" -o r ./a.out -- hello world
    1. The -b toggles on batch mode. I find this handy because it lets you execute your program in a fire-and-forget fashion without leaving you in the debugger when your program is done.
    2. The -o "br set -X foo -p return" sets a breakpoint on the pattern return inside the function foo. Note that a breakpoint is limited only to the return statements inside the function foo (we have four, not five locations).
    3. The -o r runs the program and stops at the breakpoint inside foo.
    4. Everything after the -- is passed to our program ./a,out as arguments.

    Here you see the effect:

    (lldb) target create "./a.out"
    Current executable set to './a.out' (x86_64).
    (lldb) settings set -- target.run-args  "hello" "world"
    (lldb) br set -X foo -p return
    Breakpoint 1: 4 locations.
    (lldb) r
    Process 7542 stopped
    * thread #1, name = 'a.out', stop reason = breakpoint 1.3
        frame #0: 0x0000000000401170 a.out`foo(argc=3) at test.cpp:8:5
       5   	  case 2:
       6   	    return 2;
       7   	  case 3:
    -> 8   	    return 3;
       9   	  }
       10  	  return -1;
       11  	}
    
    Process 7542 launched: '/home/kkleine/a.out' (x86_64)
    

    I hope you like this tip. For more useful LLDB tips on breakpoints, please visit this page: https://lldb.llvm.org/use/tutorial.html#setting-breakpoints

    Last updated: July 1, 2020

    Recent Posts

    • Create and enrich ServiceNow ITSM tickets with Ansible Automation Platform

    • Expand Model-as-a-Service for secure enterprise AI

    • OpenShift LACP bonding performance expectations

    • Build container images in CI/CD with Tekton and Buildpacks

    • How to deploy OpenShift AI & Service Mesh 3 on one cluster

    Red Hat Developers logo LinkedIn YouTube Twitter Facebook

    Products

    • Red Hat Enterprise Linux
    • Red Hat OpenShift
    • Red Hat Ansible Automation Platform

    Build

    • Developer Sandbox
    • Developer Tools
    • Interactive Tutorials
    • API Catalog

    Quicklinks

    • Learning Resources
    • E-books
    • Cheat Sheets
    • Blog
    • Events
    • Newsletter

    Communicate

    • About us
    • Contact sales
    • Find a partner
    • Report a website issue
    • Site Status Dashboard
    • Report a security problem

    RED HAT DEVELOPER

    Build here. Go anywhere.

    We serve the builders. The problem solvers who create careers with code.

    Join us if you’re a developer, software engineer, web designer, front-end designer, UX designer, computer scientist, architect, tester, product manager, project manager or team lead.

    Sign me up

    Red Hat legal and privacy links

    • About Red Hat
    • Jobs
    • Events
    • Locations
    • Contact Red Hat
    • Red Hat Blog
    • Inclusion at Red Hat
    • Cool Stuff Store
    • Red Hat Summit
    © 2025 Red Hat

    Red Hat legal and privacy links

    • Privacy statement
    • Terms of use
    • All policies and guidelines
    • Digital accessibility

    Report a website issue